Project Summary/Abstract The overarching hypothesis of this project is that peripheral immune dysregulation, independent of central nervous system (CNS) immune dysregulation, is a critical driver of cognitive decline, and this relationship is exacerbated by higher Alzheimer's Disease (AD)-related pathology and by recent immune health events. Epidemiological studies indicate that elevated levels of peripheral immune markers are evident years prior to clinical manifestation of Alzheimer's Disease. Although peripheral immune dysfunction is a risk factor for cognitive decline, many key unanswered how to effectively measure peripheral and CNS immune changes; 2) how to disentangle the mechanistic role of peripheral and CNS immune dysregulation on aging outcomes, and 3) how to evaluate the effect of immune-related health events on cognitive decline. To address gaps in the field, we propose to use longitudinal, multimodal measurements of peripheral and CNS immune markers. In addition to examining immune markers in circulating blood and CSF, we will employ a cutting-edge method to analyze immune markers carried by periphery- and CNS- derived extracellular vesicles (EVs). EVs play key roles in cell-to-cell communication and EV methodologies also allow for specification of the originating cells. Our primary aims for this study are: to determine how peripheral immune markers relate to CNS immune markers over time (Aim 1); to determine whether peripheral and CNS immune markers show independent associations with cognitive outcomes, and to delineate how AD- related pathology influences these associations (Aim 2); and to evaluate the role of immune health history on the relationship between peripheral and CNS immune markers and cognitive decline (Aim 3). To test our hypotheses, we will prospectively assess 180 AOAs enriched for preclinical AD (i.e., AOAs with positive amyloid PET brain scans) with baseline and 24-month follow-up measurements of peripheral immune markers (i.e., in circulating blood and in periphery-derived blood EVs) and of CNS immune markers (i.e., in CSF and in CNS-derived blood EVs), and correlate these markers with metrics of cognition and biomarkers of AD (i.e., amyloid and p-tau). We will employ both a targeted and an exploratory immune marker panel, and will record recent immune health events to determine their potential impact on the relationship between immune dysregulation and cognitive decline. Importantly, this study will establish the most comprehensively immunophenotyped aging cohort to date and will allow us to address critical questions regarding the role of the peripheral immune system in typical and pathological aging. By determining the contribution(s) of peripheral- versus CNS-derived immune signals to cognitive aging trajectories, we will be poised to better understand, predict, and ultimately treat early pathogenic immune events that may drive AD pathogenesis. questions remain, including: 1)